I don't know if this is what you need to know, but I will tell you what I do:
If I have a book or document that I want on my Kindle, I convert it to mobi format (with Calibre) and then send it to my kindle at the "free" email address.
I have not done it yet, but I suppose that if I had a document that I could not convert, then I might email it to the "non-free" address and see if Amazon could do the appropriate conversion (for a fee) and send it to me.
Be it right or wrong, that's what I do and it works fine for me.

BTW: sideload via USB means to plug your K3 to your computer via the USB cable and do the transfers that way.

But what do you mean by "sideload via USB" ? I don't think USB is needed here.

The email service that Amazon runs also does file conversion for you if you don't wish to do it yourself using software such as Calibre.

You email the file to them and they'll automaticially deliver the file if you turn on your Kindle and enable wireless. They also send you an email reply containing a URL that lets you download and transfer the file across to your Kindle manually using USB.

I have not done it yet, but I suppose that if I had a document that I could not convert, then I might email it to the "non-free" address and see if Amazon could do the appropriate conversion (for a fee) and send it to me.

The conversion is the same and free for both the regular @kindle.com address and @free.kindle.com address. It is the actual 3G delivery that costs money. The conversion service, USB sideloading and WiFi delivery are all free and don't cost anything.

That tends to get confusing. My understanding is that if you have a 3g Kindle, you should use the @free.kindle.com address in order to get the free transfer. If you have wifi only, @kindle.com is all you need and is free.

"In general, send personal documents to your "name"@free.kindle.com address to wirelessly transfer personal documents to your Kindle over Wi-Fi as well as to the e-mail address associated with your Amazon.com account at no charge.

If you are not able to connect your Kindle via Wi-Fi, send your documents to your "name"@kindle.com address. The files will be sent to your Kindle over Wi-Fi if available. If Wi-Fi is not available, the files will be sent via 3G for a small fee."

I'm not sure what qualifies as a personal document. If I buy a book at a store other than Amazon and send it to my Kindle, is that considered a personal document?

If so, I've done that a number of times without charge on my wifi K3 using the @kindle.com email address.

I'm not sure what qualifies as a personal document. If I buy a book at a store other than Amazon and send it to my Kindle, is that considered a personal document? If so, I've done that a number of times without charge on my wifi K3 using the @kindle.com email address.

There are websites like Project Gutenberg where you can access the website from your Kindle browser and just download the kindle file straight on to your Kindle - you don't input any email at all to do this. And it is free to download the file.

Any site which requires that you add their email onto your Manage Your Kindle page on the Amazon store and also needs your Kindle email to deliver files is classed as sending your Kindle "personal documents" and that is where those who have a 3G enabled Kindle need to be mindful that a small fee may be involved. So either set your personal document limit to 0.00 or use the free kindle address in this instance if you don't want to risk getting charged.

I found this just now from another post just down the list:
************
If you have a Wifi+3G, and you send it to the @free.kindle.com, it will only download once you're in range of Wifi. If you're out and about, you won't get the delivery.

If you have the WIfi+3G and you send it to the @kindle.com addy, it will download via Wifi if you're in range, for free, or via the cellular connection if you're away from wifi, in which case it charges you.
*************

Since I have a wifi only K3, I can only d/l when in range of a hotspot so I'll never get charged and don't need to worry about setting my max fee to $0.00.

I should have clarified that I d/l my non-Amazon purchases to my PC and via calibre, send them via email to my K3 using the @kindle.com address. I only sideload if the filesize is too big to send via email.

The conversion is the same and free for both the regular @kindle.com address and @free.kindle.com address. It is the actual 3G delivery that costs money. The conversion service, USB sideloading and WiFi delivery are all free and don't cost anything.